This project will develop new techniques and machinery for rapidly freezing isolated muscles as they are stimulated and monitored electrophysiologically. Neuromuscular junctions frozen at the precise moment of transmission will be prepared by freeze-fracture and freeze-substitution for examination in the electron microscope. Thus, the fleeting structural changes that occur during acetylcholine secretion from nerve and reception by muscle will be revealed in their natural form. This structural data should help to elucidate the molecular mechanisms that underlie normal neuromuscular transmission, and form a basis of comparison with neuromuscular pathology.